1. The Field of the Invention
The present invention relates to catalysts for use in various chemical processes. More particularly, the invention relates to highly selective supported catalysts having a controlled coordination structure and methods of manufacturing such catalysts.
2. The Relevant Technology
Catalysts are widely used in many industries including chemical, petroleum, pharmaceutical, energy, and automotive. Many of the catalysts used in these industries are based on dispersed particles of certain active components, where the active components are commonly metals or combinations of metals and other elements. In catalysts of this type, the catalytic properties of the materials are determined by both the type of active components selected, i.e., the elemental composition of the catalyst, and the detailed structure of the dispersed particles, i.e., the atomic scale structure and orientation of the dispersed particles.
Historically, much of the work in the development and optimization of catalysts has focused on the selection of the appropriate catalytic components. Prior methods have allowed catalyst developers to control the selection and relative amounts of catalyst components. However, the control of the detailed structure of catalysts, particularly on the atomic scale, has presented a much greater difficulty. Controlling the atomic scale structure can be as important in the development of effective catalysts as selecting the elemental composition. For example, control of the detailed catalyst crystal structure can relate directly to the selectivity of the catalyst. A method which allowed for the controlled exposure of certain kinds of catalytic active sites would allow certain reaction pathways to be favored to an extent that is not currently possible using a catalyst that contains a mixture of different types of active sites.
One particularly useful way of defining a preferred catalytic structure is based on the geometry of the surface active sites. Because of thermodynamic considerations, it is normally the case that particles of crystalline materials will expose one or more of a limited number of low-index crystal faces. Common low-index crystal face exposures of metal particles include, for example, the 111, 100, and 110 crystal faces of the common crystal lattices, which include face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). Exemplary crystal faces are schematically illustrated in FIG. 1. Each of these crystal faces has a different arrangement of atoms, and may therefore display different catalytic properties with respect to certain chemical reactions. Therefore, substantial improvements in catalyst function could theoretically be achieved if a method were available to exert effective control over the atomic-scale structure of catalytic particles. A more detailed description of metal crystal surface structure can be found by accessing the National Institute of Standards and Technology (NIST) WWW home page, particularly the Surface Structure Database (SSD).
Despite the extensive history of catalyst development, there are few, if any, reliable methods which allow the detailed crystal structure of dispersed catalytic particles to be controlled as a way of improving and optimizing catalytic function. In part, this derives from the intrinsic difficulty of controlling structures at an atomic scale. It is also related to a lack of methods to accurately determine whether a desired atomic scale structure has been successfully achieved. Moreover, as difficult as it might be to control the shape of the catalyst crystal lattice, one of skill in the art would find it even more difficult to control the crystal face exposure of a catalyst crystal.
Attempts have been made to control the crystal lattice structure of active catalyst particles. An article by Termer S. Ahmadi, et al. of Georgia Institute of Technology, entitled “Shape-Controlled Synthesis of Colloidal Platinum Nanoparticles”, published in Science, Vol. 272, pp. 1924–26, describes a method for the synthesis of shape-controlled platinum particles by controlling the ratio of the concentration of shaping material to that of ionic platinum. “Tetrahedral, cubic, irregular-prismatic, icoshedral, and cubo-octahedral particle shapes were observed, whose distribution was dependent on the concentration ratio of the capping polymer material to the platinum cation.” Id, p. 1924. The article is silent, however, with respect to how to control crystal face exposure of a given crystal shape. Moreover, the article not only fails to teach how to select or increase the preponderance of one crystal face exposure of a catalyst crystal structure over another, it provides no teaching or suggestion that would motivate the selection of any particular crystal face exposure over another.